This application is a 371 of PCT/JP00/07949 filed Nov. 10, 2000.
The present invention relates to a photosensitive composition, an optical waveguide element comprising the same and a process for producing the optical waveguide element.
Along with the rapid spread of the Internet and multi-media, a need for optical communication systems having higher speed and larger capacity has recently been growing. As a means of meeting the need, much attention is being paid to a wavelength multiplex communication system (WDM) which eliminates the need of increasing the number of optical fibers and makes use of the existing optical fibers. Optical parts and materials are necessary for the construction of this system. Among the optical parts are an optical waveguide element, an optical filter element and the like. For these optical parts and materials for optical communication, the supply of a material which needs to have transmission at communication wavelength ranges and such characteristic properties as heat resistance, water resistance and chemical resistance, and makes it possible to produce them and incorporate them in a mounting module with ease is desired.
The following materials (i) to (vi) are known as the material which satisfies the above requirements:
(i) a coating solution composition for forming an optical material which comprises a trifunctional silane and metal alkoxide, and an optical material formed therefrom (JP-A 7-331173) (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d),
(ii) a resin material for optical transmission lines which is a partial hydrolysis and condensation polysiloxane resin of deuterated tetraalkoxysilane and an optical waveguide formed therefrom (JP-A 8-313742),
(iii) an optical waveguide element having a grating manufactured from a photosensitive material comprising methacryloxypropyl trimethoxysilane, zirconium alkoxide and acrylic acid (JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 16, No. 9, pp. 1640-1646, September 1998, SPIE Vol. 3282, pp. 17-30 and SPIE Vol. 3282, pp. 50-58)
(iv) a polymer optical waveguide element having a low loss at a communication wavelength (Optonews, No. 2, pp. 31-32, 1999),
(v) an organic-inorganic hybrid material having excellent transparency and comprising methacryloxypropylmethyl dichlorosilane (MPMDCS), 1H, 1H, 2H, 2H-tridecafluorooctylmethyl dichlorosilane (FOMDCS) and heptadecafluorodecyl methacrylate (HFDMA) (M. Menning, M. Zahnhausen, H. Schmidt, Proc. SPIE Vol. 3469, pp. 68-78, 1998) (The Cxe2x80x94H group of MPMDCS which is a photosensitive raw material remains unchanged and FOMDCS having a large number of Cxe2x80x94F groups and HFDMA which is a photosensitive organic monomer are added excessively to reduce the number of the Cxe2x80x94H groups), and
(vi) an organic-inorganic hybrid material which comprises tetraalkoxysilane, alkyltrialkoxysilane and aryltrialkoxysilane (WO 98/25862) (This publication shows that the absorption of IR having a wavelength of 1.3 xcexcm and a wavelength of 1.553 xcexcm derived from the Cxe2x80x94H bond can be reduced by using CD3Si(OC2H5)3 and C6D5Si(OC2H5)3).
However, the above prior arts have the following problems. First, since the above material (i) has an alkyl group and alkoxy group and its absorbed harmonic component based on the Cxe2x80x94H bond is at a near infrared range, it cannot be said that it has high transmission at communication ranges of 1.55 xcexcm and 1.3 xcexcm.
The material (ii) has such a problem that when a film having a thickness required for the formation of an optical waveguide element is to be formed therefrom, the film is easily cracked although the loss of light having communication wavelengths is reduced by using deuterated tetraalkoxysilane. Further, it is not easy to form a diffraction grating on the optical waveguide.
Since the element (iii) is made from a photosensitive material, an optical waveguide having a grating can be easily manufactured by a method such as optical processing. However, like the above material (i), it cannot be said that the element has high transmission at communication ranges. As the element (iv) is made from a polymer, it does not always have satisfactory workability, reliability for parts and stability.
The above organic-inorganic hybrid material (v) must contain an organic component having a Cxe2x80x94F group in excess to ensure light transmission and only an organic-inorganic hybrid material having an extremely small content of an inorganic component is provided. The content of the inorganic component cannot be increased in full so as to improve thermal characteristics (environmental resistance and dimensional stability obtained by reducing thermal expansion coefficient).
The above material (vi) cannot be used as a material to be processed by photolithography for the manufacture of a diffraction grating by double-beam interference exposure, the manufacture of an optical waveguide by exposure using a photomask and patterning by the leaching of an unexposed portion, or the manufacture of a diffraction grating by phase mask exposure because the alkyl group (methyl group) or aryl group (phenyl group) contained in CD3Si(OC2H5)3 and C6D5Si(OC2H5)3 does not have photopolymerizability or thermopolymerizability.
The present invention has been made in view of the above problems of the prior art. That is, it is an object of the present invention to provide a photosensitive composition which has high transmission at wavelength ranges used for communication, excellent characteristic properties such as heat resistance, water resistance and chemical resistance and is used as a process material which makes it possible to form a grating and incorporate it in a mounting module with ease. It is another object of the present invention to provide a photosensitive composition for forming a light transmitting material which enables the free control of the proportions of an organic component and an inorganic component and fine light processing through exposure and has a photosensitive organic group.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a photosensitive composition (may be referred to as xe2x80x9ccomposition Axe2x80x9d hereinafter) for forming a light transmitting material, comprising a silane compound or hydrolysis/dehydration condensation reaction product thereof, a photoinitiator and water, wherein
the silane compound is represented by the following formula (1):
R1SiX13xe2x80x83xe2x80x83(1)
wherein R1 is an organic group having a polymerizable carbon-carbon double bond, and X1 is a hydrolyzable group or atom, with the proviso that at least 40% of the total number of hydrogen atoms of the organic group R1 are substituted by at least one substituent atom selected from the group consisting of deuterium, fluorine, chlorine and bromine.
According to the present invention, secondly, the above objects and advantages of the present invention are attained by a photosensitive composition (may be referred to as xe2x80x9ccomposition Bxe2x80x9d hereinafter) for forming a light transmitting material, comprising:
(A) a metal compound represented by the following formula (2) or a hydrolysis/dehydration condensation reaction product thereof:
M1X24xe2x80x83xe2x80x83(2)
wherein M1 is Si, Al, Zr, Ge or Ti, and X2 is a hydrolyzable group or atom;
(B) acrylic acid, methacrylic acid or acid ester thereof represented by the following formula (3): 
wherein R2 is a hydrogen atom, alkyl group or ketoalkyl group, and Y is a hydrogen atom or methyl group, with the proviso that at least 40% of the total number of hydrogen atoms in the formula (3) are substituted by at least one substituent atom selected from the group consisting of deuterium, fluorine, chlorine and bromine;
(C) a photoinitiator; and
(D) water.
According to the present invention, thirdly, the above objects and advantages of the present invention are attained by a photosensitive composition (may be referred to as xe2x80x9ccomposition Cxe2x80x9d hereinafter) for forming a light transmitting material, comprising:
(Axe2x80x2) a silane compound represented by the following formula (4) or a hydrolysis/dehydration condensation reaction product thereof:
R3SiX33xe2x80x83xe2x80x83(4)
wherein R3 is an organic group having a polymerizable carbon-carbon double bond, and X3 is a hydrolyzable group or atom, with the proviso that some of the hydrogen atoms of the organic group R3 may be substituted by at least one substituent atom selected from the group consisting of deuterium, fluorine, chlorine and bromine;
(Bxe2x80x2) a metal compound represented by the following formula (5) or a hydrolysis/dehydration condensation reaction product thereof:
M2X44xe2x80x83xe2x80x83(5)
wherein M2 is Si, Al, Zr, Ge or Ti, and X4 is a hydrolyzable group or atom;
(Cxe2x80x2) acrylic acid, methacrylic acid or acid ester thereof represented by the following formula (6): 
wherein R4 is a hydrogen atom, alkyl group or ketoalkyl group, and Z is a hydrogen atom or methyl group, with the proviso that some of the hydrogen atoms in the formula (6) may be substituted by at least one substituent atom selected from the group consisting of deuterium, fluorine, chlorine and bromine;
(Dxe2x80x2) a photoinitiator; and
(Exe2x80x2) water, wherein
the component (Axe2x80x2) and the component (Cxe2x80x2) are used such that the above substituent atom(s) account(s) for at least 40% of the total of the hydrogen atoms of the organic group R3 in the formula (4) and the hydrogen atoms in the formula (6).
According to the present invention, in the fourth place, the above objects and advantages of the present invention are attained by an optical waveguide element formed from the above composition A, B or C of the present invention.
According to the present invention, in the fifth place, the above objects and advantages of the present invention and attained by a process for producing an optical waveguide element comprising the steps of applying the photosensitive composition for forming a light transmitting material of the present invention to the surface of a substrate at least the surface layer of which has a low refractive index to form a film, exposing the film to ultraviolet radiation through a photomask placed upon the film, dissolving an unexposed film portion in a solvent to remove the portion according to circumstances, and thermally curing the film to form a core.
The present invention will be described in detail hereinunder.
The silane compound represented by the above formula (1) in the composition A will be described. In the above formula (1). R1 is an organic group having a polymerizable carbon-carbon double bond and X1 is a hydrolyzable group or atom. Examples of X1 include an alkoxyl group, alkenyloxy group and halogen atom. The organic group R1 may be linear, branched or cyclic. It is preferably an organic group having a carbon-carbon double bond and 1 to 10 carbon atoms. The organic group R1 is preferably an alkenyl group. This organic group is photosensitive.
The organic group is preferably an alkyl group or aryl group modified by vinyl group, vinyloxy group, allyl group, allyloxy group, acryl group, acryloxy group, methacryl group or methacryloxy group. Examples of the above modified alkyl group and aryl group include vinylalkyl groups such as vinylmethyl group and vinylethyl group; vinyloxyalkyl groups such as vinyloxymethyl group and vinyloxyethyl group; vinyl group, alkenyl group such as allyl group, acryl group and methacryl group; vinylaryl groups such as vinylphenyl group and vinyltolyl group; vinyloxyaryl groups such as vinyloxyphenyl group and vinyloxytolyl group; allylalkyl groups such as allylmethyl group and allylethyl group; allyloxyalkyl groups such as allyloxymethyl group and allyloxyethyl group; allylaryl groups such as allylphenyl group and allyltolyl group; allyloxyaryl groups such as allyloxyphenyl group and allyloxytolyl group; allylamino group; acrylalkyl groups such as acrylmethyl group and acrylethyl group; acryloxyalkyl groups such as acryloxymethyl group and acryloxyethyl group; methacrylalkyl groups such as methacrylmethyl group and methacrylethyl group; methacryloxyalkyl groups such as methacryloxymethyl group and methacryloxyethyl group; acrylaryl groups such as acrylphenyl group and acryltolyl group; acryloxyaryl groups such as acryloxyphenyl group and acryloxytolyl group; methacrylaryl groups such as methacrylphenyl group and methacryltolyl group; methacryloxyaryl groups such as methacryloxyphenyl group and methacryloxytolyl group; bicycloheptenyl group and (acryloxymethyl)phenethyl group. Out of these, methacryloxyalkyl groups are particularly preferred.
The alkoxyl group and alkenyloxy group as the hydrolyzable group represented by X1 in the above formula (1) may be linear or branched and preferably have 1 to 4 carbon atoms. Examples of the halogen atom as the hydrolyzable atom represented by X1 include fluorine, chlorine and bromine. Chlorine and fluorine are preferred.
Examples of the silane compound represented by the above formula (1) include 3-methacryloxyalkyl trialkoxysilane, 3-methacryloxyalkyl trichlorosilane, vinyltrimethoxysilane, vinyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, 3-acryloxypropyltrichlorosilane, 3-(N-allylamino)propyltrimethoxysilane, allylaminotrimethoxysilane, (2-cyclohexenyl-2-ethyl)trialkoxysilane, allyltrialkoxysilane, 5-(bicycloheptenyl)trialkoxysilane, (acryloxymethyl)phenethyltrialkoxysilane, 1,1-bis(trialkoxysilylmethyl)ethylene, bis(triethoxysilyl)ethylene, bis(triethoxysilyl)-1,7-octanediene, butenyltriethoxysilane, 1-chloro-2-methylallyltrichlorosilane, 2-(chloromethyl)allyltrichlorosilane, [2-(3-cyclohexenyl)ethyl]trichlorosilane, 3-cyclohexenyltrichlorosilane, (4-cyclooctenyl)trichlorosilane, (3-cyclopentadienylpropyl)triethoxysilane, 5-hexenyltrialkoxysilane, O-(methacryloxyethyl)-N-(triethoxysilylpropyl)urethane, (2-methyl-propenyl)trimethylsilane, 1,7-octadienyltriethoxysilane, 7-octenyltrialkoxysilane, (2,4-pentadienyl)trialkoxysilane, styrylethyltrimethoxysilane and vinyltriisopropenoxysilane. Out of these, methacryloxyalkyltrialkoxysilane is particularly preferred.
In the composition A of the present invention, the silane compound represented by the above formula (1) is such that at least 40% of the total number of the hydrogen atoms of the organic group R1 in the formula (1) are substituted by at least one substituent atom selected from the group consisting of deuterium (hydrogen isotope having a mass number of 2 or 3, may be abbreviated as D hereinafter), fluorine (F), chlorine (Cl) and bromine (Br). Preferably at least 60%, more preferably at least 80%, the particularly preferably 100% of the total number of the hydrogen atoms are substituted by the above substituent atom. Out of the above substituent atoms, deuterium is particularly preferred.
Since a small amount of an alkoxyl group or alkenyloxy group may remain in the film when X1 in the above formula (1) is an alkoxyl group or alkenyloxy group, X1 is preferably an alkoxyl group or alkenyloxy group substituted by deuterium.
The silane compound represented by the above formula (1) (may be referred to as xe2x80x9ccomponent (1)xe2x80x9d hereinafter) is, for example, deuterated (3-methacryloxypropyltrimethoxysilane-d11) represented by the following formula (7), deuterated (3-methacryloxypropyltriethoxysilane-d26) represented by the following formula (8), deuterated (3-methacryloxyethyltrimethoxysilane-d18) represented by the following formula (9), deuterated (3-methacryloxypropyltrimethoxysilane-d5) represented by the following formula (10), deuterated and fluorinated (3-methacryloxyethyltriethoxysilane-d20) represented by the following formula (11), deuterated allyltrimethoxysilane-d13 represented by the following formula (12), deuterated vinyltrimethoxysilane-d11 represented by the following formula (13) or fluorinated alkyltrialkoxysilane represented by the following formula (14). xe2x80x83CD2xe2x95x90CDxe2x80x94CD2xe2x80x94Si(OCD3)3xe2x80x83xe2x80x83(12)
CD2xe2x95x90CDxe2x80x94Si(OCD3)3xe2x80x83xe2x80x83(13)
CH2xe2x95x90CHxe2x80x94(CF2)nxe2x80x94CH2xe2x80x94CH2xe2x80x94SiX3xe2x80x83xe2x80x83(14)
(in the formula (14), n=2 to 15, and X is an alkoxy group such as methoxy group or ethoxy group, with the proviso that the hydrogen atom of the alkoxyl group may be substituted by a deuterium atom.)
The composition A of the present invention contains the above compound or hydrolysis/dehydration condensation reaction product thereof. When the degree of condensation of the condensation reaction product of this compound is too high, precipitation easily occurs. The degree of condensation is preferably 500 or less. The composition A contains a photointiator and water and optionally a solvent and a hydrolytic catalyst in addition to the compound of the formula (1) or hydrolysis/dehydration condensation reaction product thereof.
Water is required to complete the hydrolysis/dehydration condensation reaction of the above compound or hydrolysis/dehydration condensation reaction product thereof. Ordinary water (H2O) may be used but it may remain in the obtained light transmitting material in a trace amount. In this case, a wavelength range absorbed by the OH group oscillation may overlap with communication wavelength ranges, thereby preventing light transmission. Therefore, heavy water (D2O) is preferably used. Water is preferably added in a stoichiometric amount required for hydrolysis or more. When the amount of water is smaller than the stoichiometric amount, unreacted alkoxysilane remains at the time of a heat treatment for gelation. The amount of water is generally 0.8 to 30 times the required stoichiometric amount including water of an aqueous catalyst solution and the molar ratio of water to the component (1) is preferably 1:1 to 20:1, more preferably 2:1 to 10:1.
The solvent is preferably an alcohol. The alcohol is preferably a lower alcohol having 1 to 4 carbon atoms, particularly preferably methanol or ethanol having a low boiling point. The reason for this is that the alcohol can be removed rapidly from the solution by a heat treatment at a relatively low temperature after hydrolysis. The molar ratio of the alcohol to the component (1) is preferably 0.3:1 to 5:1, more preferably 0.5:1 to 1.5:1. To obtain a high light transmission at communication wavelength ranges, an alcohol having a deuterium atom substituted for a hydrogen atom is preferably used. Since the alcohol may be formed by the above hydrolytic reaction, it is not necessarily an essential component.
The catalyst is preferably an acid catalyst. The acid catalyst is preferably used in the form of an aqueous solution of at least one acid catalyst selected from formic acid, acetic acid, propionic acid, oxalic acid, hydrochloric acid, nitric acid and sulfuric acid. In this case, to obtain a high light transmission at communication wavelength ranges, an acid containing a deuterium atom substituted for a hydrogen atom is more preferably used. The amount of the acid catalyst differs according to the type of the acid and the strength of protonic acid (weak acid or strong acid). When the amount is too small, the proceeding of a hydrolysis/dehydration condensation reaction slows down and when the amount is too large, a condensation reaction proceeds too far, whereby the molecular weight becomes too large and the gelation of a precipitate and a coating solution readily occurs disadvantageously. Therefore, the amount of the acid catalyst added is preferably 0.01 to 10 mmol, more preferably 0.05 to 7 mmol based on 1 mol of the component (1) when hydrochloric acid is used as the acid catalyst.
The photoinitiator promotes photopolymerization when the organic group having a polymerizable carbon-carbon double bond contained in the composition A is exposed to light. Examples of the photoinitiator include 1-hydroxycyclohexyl-1-phenylketone (Irgacure 184 of CIBA Co., Ltd.), 2-hydroxy-2-methyl-1-phenylpropan-1-one (Darocure 1173 of Merk Co., Ltd.), 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one (Darocure 1116 of Merk Co., Ltd.) and 2,2-dimethoxy-2-phenylacetophenone (Irganocure 651 of CIBA Co., Ltd.). The amount of the photoinitiator is 0.001 to 0.2 mol based on 1 mol of the component (1) or 0.1 to 20 wt % based on the composition A.
Various additives may be contained in the composition A. Among the additives are a film thickness increasing agent, thickener, leveling agent and flow control agent, as exemplified by silicones such as dimethylpolysiloxane and glycols such as polyethylene glycol. The additives are also preferably those whose hydrogen atom is substituted by an atom selected from the group consisting of deuterium, fluorine, chlorine and bromine.
A description is subsequently given of the composition B. In the formula (2), M1 is Si, Al, Zr, Ge or Ti, and X2 is a hydrolyzable group or atom. X2 is, for example, an alkoxyl group, alkenyloxy group or halogen atom. The alkoxyl group and alkenyloxy group as the hydrolyzable group X2 may be linear or branched and preferably have 1 to 4 carbon atoms. The halogen atom as the hydrolyzable atom X2 is, for example, fluorine, chlorine or bromine, out of which chlorine and fluorine are preferred. The alkoxyl group and alkenyloxy group represented by X2 are preferably an alkoxyl group and alkenyloxy group substituted by deuterium, respectively. Examples of the metal compound (maybe referred to as xe2x80x9ccomponent (2)xe2x80x9d hereinafter) represented by the above formula (2) include tetramethoxysilane, tetraethoxysilane, tetrabutoxyaluminum, tetrapropoxyzirconium, tetrabutoxyzirconium, tetraisopropoxytitanium, tetrabutoxytitanium, tetrachlorosilane, tetrachloroaluminum, tetrachlorozirconium, tetrachlorotitanium, tetramethoxygermane, tetraethoxygermane and tetraisopropoxygermane. A different metal alkoxide such as di-s-butoxyaluminoxytriethoxysilane may also be used. Out of these, the metal compound having an alkoxyl group is preferably a compound at least 40% (preferably at least 60%, more preferably at least 80%, the most preferably 100%) of the total number of hydrogen atoms of the alkoxyl group of which are substituted by at least one elemental atom selected from the group consisting of deuterium, fluorine, chlorine and bromine. Examples of the compound are deuterated tetramethoxysilane represented by the following formula (15), deuterated tetraethoxysilane represented by the following formula (16), deuterated tetrabutoxyaluminum represented by the following formula (17), deuterated tetrapropoxyzirconium represented by the following formula (18), deuterated tetrabutoxyzirconium represented by the following formula (19), deuterated tetraisopropoxytitanium represented by the following formula (20), deuterated tetrabutoxytitanium represented by the following formula (21), deuterated tetramethoxygermane represented by the following formula (22) and deuterated tetraethoxygermane represented by the following formula (23).
Si(OCD3)4xe2x80x83xe2x80x83(15)
Si(OCD2CD3)4xe2x80x83xe2x80x83(16)
Al(OCD2CD2CD2CD3)4xe2x80x83xe2x80x83(17)
Zr(OCD2CD2CD3)4xe2x80x83xe2x80x83(18)
Zr(OCD2CD2CD2CD3)4xe2x80x83xe2x80x83(19)
Ti(OCD(CD3)2)4xe2x80x83xe2x80x83(20)
Ti(OCD2CD2CD2CD3)4xe2x80x83xe2x80x83(21)
Ge(OCD3)4xe2x80x83xe2x80x83(22)
Ge(OCD2CD3)4xe2x80x83xe2x80x83(23)
The metal compound (component (2)) represented by the formula (2) may be contained in the composition B as it is, or may be contained in the composition B as a hydrolysis/dehydration condensation reaction product. In the latter case, when the degree of condensation of the condensation reaction product is too high, precipitation occurs. Therefore, the degree of condensation is preferably 500 or less.
The photosensitive (meth)acrylic compound represented by the above formula (3) used in the composition B of the present invention will be described hereinbelow. The alkyl group represented by R2 in the formula (3) is preferably an alkyl group having 1 to 20 carbon atoms and the ketoalkyl group represented by R2 is, for example, xe2x80x94CH2(CO)CH2(CO)CH3. Examples of the (meth)acrylic compound represented by the formula (3) include methacrylic acid, acrylic acid, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate and methacryloxyacetyl acetonate.
The acrylic acid, methacrylic acid or acid ester thereof represented by the formula (3) in the present invention (may be referred to as xe2x80x9ccomponent (3)xe2x80x9d hereinafter) is such that at least 40%, preferably at least 60%, more preferably at least 80%, the most preferably 100% of the total number of hydrogen atoms in the formula (3) are substituted by at least one substituent atom selected from the group consisting of deuterium, fluorine, chlorine and bromine. Examples of the compound include deuterated and fluorinated methyl methacrylate represented by the following formula (24), deuterated and fluorinated ethyl methacrylate represented by the following formula (25), deuterated and fluorinated propylmethacrylate represented by the following formula (26), deuterated and fluorinated butyl methacrylate represented by the following formula (27), methyl pentafluoromethacrylate represented by the following formula (28) and deuterated butyl methacrylate represented by the following formula (29). 
When M1 of the compound represented by the above formula (2) is Al, Zr, Ge or Ti, the component (3) desirably stabilizes the component (2) to control the hydrolysis/condensation reaction of the compound of the formula (2). The component (3) is, for example, a carboxylic acid compound or chelating reagent. Examples of the compound include deuterated methacrylic acid and deuterated methacryloxydiacetylacetone compound. More specifically, they are compounds represented by the following formulas (30) to (33). 
As for the composition B, the amounts of the component (2) and the component (3) will be described hereinbelow. The component (2) is a raw material which has excellent environmental resistance and heat resistance and gives an inorganic component for increasing refractive index whereas the component (3) gives flexibility to a film and photosensitivity to a material composition and has the function of reducing the refractive index of a film. Therefore, when the amount of the component (2) is too large, the flexibility of a film and the photosensitivity of a material composition are lost disadvantageously. When the amount of the component (3) is too large, environmental resistance and heat resistance are lost and the controllable range of refractive index is narrowed disadvantageously. The amount of the component (2) is in the range of preferably 25 to 75 mol %, more preferably 30 to 70 mol %, much more preferably 40 to 60 mol %. The amount of the component (3) is in the range of preferably 25 to 75 mol %, more preferably 30 to 70 mol %, much more preferably 40 to 60 mol %. The term xe2x80x9cmol %xe2x80x9d is based on the total number of mols of the component (2) and the component (3). The number of mols of the component (2) is based on the number of mols of the metal compound represented by the formula (2).
A description is subsequently given of each component of the composition C. The silane compound (may be referred to as xe2x80x9ccomponent (4)xe2x80x9d hereinafter) represented by the above formula (4) will be first described. In the formula (4), R3 is an organic group having a polymerizable carbon-carbon double bond, and X3 is a hydrolyzable group or atom. Examples of the organic group and the hydrolyzable group or atom are the same as those enumerated for the above formula (1). In the formula (4), some or all of the hydrogen atoms of the above organic group may be substituted by a substituent atom selected from the group consisting of deuterium, fluorine, chlorine and bromine and does not always need to be substituted by the substituent atom unlike the formula (1). The component (4) gives flexibility to a film and photosensitivity to a material composition and has the function of reducing the refractive index of a film.
The metal compound represented by the formula (5) (may be referred to as xe2x80x9ccomponent (5)xe2x80x9d hereinafter) may be identical to the metal compound represented by the above formula (2) which has been described for the composition B. This component has excellent environment resistance and heat resistance and is a raw material which gives an inorganic component for increasing refractive index.
A description is subsequently given of the acrylic acid, methacrylic acid or acid ester thereof represented by the formula (6) (maybe referred to as xe2x80x9ccomponent (6)xe2x80x9d hereinafter). In the formula (6), R4 is a hydrogen atom, alkyl group or ketoalkyl group, and Z is a hydrogen atom or methyl group. Examples of the alkyl group and ketoalkyl group are the same as those enumerated for the above formula (3). However, some or all of the hydrogen atoms in the formula (6) may be substituted by a substituent atom selected from the group consisting of deuterium, fluorine, chlorine and bromine and does not always need to be substituted by the substituent atom unlike the formula (3). This component (5) is a raw material which gives photosensitivity to a composition and a film.
In the composition C, the above component (4) and component (6) are used such that the above substituent atom(s) account(s) for at least 40% of the total number of the hydrogen atoms of the organic group R3 in the formula (4) and the hydrogen atoms in the formula (6). A combination of the component (4) and the component (6) may be a combination of the component (4) and the component (6) all of which have a substituent atom or a combination of the component (4) and the component (6) either one of which has a substituent atom. When the component (4) has a substituent atom, it is preferred that at least 40% of the total number of hydrogen atoms of the organic group R3 in the formula (4) is substituted by a substituent atom. When the component (6) has a substituent atom, it is preferred that at least 40% of the total number of hydrogen atoms in the formula (6) is substituted by a substituent atom. The above substituent atom is preferably deuterium.
When R4 in the formula (6) is a substituent having a hydrogen atom and acetylacetone, this component is coordinately bonded to the component (5) and has a stabilization function. Therefore, it is preferred that the component (6) be existent in an amount of 1 mol or more based on 1 mol of the component (5) in this case. The amounts of the component (4), the component (5) and the component (6) are preferably in the ranges of 20 to 80 mol %, 10 to 40 mol % and 10 to 40 mol %, respectively.
The term mol % is based on the total number of mols of the component (4), the component (5) and the component (6), the number of mols of the component (4) is the number of mols of the silane compound represented by the formula (4) and the number of mols of the component (5) is the number of mols of the metal compound represented by the formula (5).
Either one or both of X3 in the formula (4) and X4 in the formula (5) are preferably an alkoxyl group or alkenyloxyl group substituted by deuterium.
The above description of the composition A is directly applied to water and a solvent which are contained in the above composition B and composition C, a photoinitiator and catalyst which are added as required, and additives which may be added. However, the amounts of water, solvent and catalyst are based on 1 mol of the component (2) in the case of the composition B and based on 1 mol of the total of the component (4) and the component (5) in the case of the composition C. The amount of the photoinitiator is based on 1 mol of the component (2) in the case of the composition B and based on 1 mol of the total of the component (4) and the component (5) in the case of the composition C.
A description is subsequently given of a process for forming an optical waveguide using the above photosensitive composition for forming a light transmitting material. The compositions A, B and C of the present invention have a polymerizable double bond. The refractive index of a composition obtained by polymerizing the above polymerizable double bond to increase the molecular weight is higher than the refractive index of a composition obtained by carrying out only dehydration polycondensation without polymerization. A core layer having a high refractive index can be formed on an exposed portion by applying a photosensitive sol-gel liquid composition containing a photoinitiator to the surface of a substrate at least the surface layer of which has a low refractive index to form a film, placing a photomask having a slit opening (ultraviolet light transmitting portion) upon the film, and exposing the film to ultraviolet radiation through the photomask. After exposure to ultraviolet radiation, a heat treatment is carried out to promote dehydration polycondensation so as to cure the film while the refractive index modulated by exposure to ultraviolet radiation is maintained. As for details of this process, please refer to New Developments in Integrated Optics Using the Sol-gel Process, P, Coudray et al, pp.286-303 and Sol-Gel and Polymer Photonic Devices, Mark P. Andrews, S. Iraj Najafi, Vol. CR68, SPIE (1997).
The exposed portion and the unexposed portion (unpolymerized portion) can be made different from each other in solubility in a solvent by polymerizing the double-bond portion exposed to ultraviolet radiation to increase the molecular weight. After exposure, the substrate is immersed in an acidic aqueous solution, alkaline aqueous solution or organic solvent to dissolve the unexposed portion, thereby making it possible to form a ridge type optical waveguide consisting of only the exposed portion (refer to 1st European Workshop on Hybrid Organic-Inorganic Materials, Nov. 8-10, 1993, New Journal of Chemistry, Vol. 18, pp.1125-1134, 1994).
An optical waveguide element or other light transmitting material is obtained by applying the above photosensitive composition (A, B or C) for forming a light transmitting material to a substrate to a wet thickness of 0.5 to 200 xcexcm in a predetermined shape so as to form a film, exposing the film to ultraviolet radiation with a light intensity of 1 to 200 mW/cm2 at an exposed position for 1 second to 2 minutes and curing the film by heating at 100 to 15xc2x0 C. for 10 minutes to 5 hours. A photomask having a predetermined local light transmission distribution is placed upon the coating film before exposure to ultraviolet radiation, the surface of the film is treated with a solvent after exposure to ultraviolet radiation to dissolve and remove an unexposed portion, and the film is cured by heating at 100 to 150xc2x0 C. for 10 minutes to 5 hours to obtain a light transmitting material having a predetermined shape.
The coating film of the photosensitive composition for forming a light transmitting material can be formed on the surface of the substrate by coating such as spin coating, dip coating, screen printing, gravure coating, flexographic printing or meniscus coating and has a uniform wet thickness, thereby making it possible to form a cured light transmitting material film having a uniform thickness.
The substrate used in the present invention may have any form such as a plate-like, bent plate-like or rod-like form. It is desired that the amount of warp of the surface of the substrate (length of thermal deformation in a direction perpendicular to the surface per unit length in the surface direction of the substrate) be small. When the amount of warp is beyond this range, the film may peel off from the substrate at the interface or crack in the step of molding a film. Therefore, the suitable material, size and shape of the substrate are preferably selected.
This substrate preferably has a linear expansion coefficient of 1.5xc3x9710xe2x88x925/xc2x0 C. or less. When the linear expansion coefficient of this substrate is larger than 1.5xc3x9710xe2x88x925/xc2x0 C., in the case of a substrate made from a plastic having a high thermal expansion coefficient of 9 to 15xc3x9710xe2x88x925/xc2x0 C. such as polypropylene, the film may peel off from the substrate at the interface or crack in the step of molding an organopolysiloxane film. Ordinary inorganic glass has a linear expansion coefficient of 1.5xc3x9710xe2x88x925/xc2x0 C. or less. At least the surface of the substrate is preferably made from an oxide. When the surface in contact with the film of the substrate is not made from an oxide, adhesion strength lowers in the step of molding a film and the film may peel off from the substrate at the interface as the case may be. Preferred examples of the material of the substrate include oxide glasses such as silicate-based glass, boric acid-based glass and phosphoric acid-based glass, quartz, ceramics, metals, epoxy resins, glass fibers and reinforced polystyrene. Although a metal is not bonded to the film as it is, when the surface of the metal is treated with an oxidizing agent in advance, it can be used as the substrate. Out of these, float glass (linear expansion coefficient: 92xc3x9710xe2x88x927/xc2x0 C.) is preferred from the viewpoint of cost, and quartz glass (linear expansion coefficient: 8xc3x9710xe2x88x927/xc2x0 C.) and zero-expansion glass (linear expansion coefficient: xe2x88x923 to 0.0xc3x9710xe2x88x927/xc2x0 C., trade name of Neocerum, Zerodua Glass) are the most preferred from the viewpoint of thermal expansion coefficient. To manufacture an integrated optical element, a silicon substrate (linear expansion coefficient: 41.5xc3x9710xe2x88x927/xc2x0 C.) may be used.
At least 40% of the total number of hydrogen atoms (excluding hydrogen atoms discharged to the outside of the light transmitting material by hydrolysis and dehydration reaction) contained in the organic group of the photosensitive composition for forming a light transmitting material of the present invention are substituted by at least one element selected from the group consisting of deuterium, fluorine, chlorine and bromine. The light absorption bands based on a Cxe2x80x94D bond, Cxe2x80x94F bond, Cxe2x80x94Cl bond and Cxe2x80x94Br bond are shifted toward a long wavelength side of that of a Cxe2x80x94H bond and away from communication bands of 1.55 xcexcm and 1.3 xcexcm. A reduction in the transmission of communication bands of 1.55 xcexcm and 1.3 xcexcm of the light transmitting material caused by absorption based on the Cxe2x80x94H bond can be prevented by the above substitution. Therefore, according to the present invention, a desired optical waveguide element or other optical element can be formed without impairing the transmission of communication bands of 1.55 xcexcm and 1.3 xcexcm. When the hydrogen atoms are substituted by fluorine, an increase in the optical loss of an optical element caused by moisture absorption can be prevented because the fluorine atom has a water-repelling effect.
A description is subsequently given of a process for fabricating a Brag diffraction grating in the optical waveguide of the present invention. The Bragg diffraction grating is formed from the photosensitive sol-gel liquid composition of the present invention as described above. As one of the methods of writing (adding) a Bragg diffraction grating to an optical waveguide by modulating refractive index and a ridge type optical waveguide by leaching after exposure, a phase mask is placed upon the optical waveguide and light from an excimer laser (wavelength: 249 nm (KrF) or 193 nm (ArF)) is irradiated onto the optical waveguide through the phase mask to induce the modulation of the Sixe2x80x94Oxe2x80x94Si binding state of a metal oxide network, for example, a silica network of a film to make a difference in refractive index so as to write a diffraction grating (refer to M. P. Andrews, SPIE vol. 3282, pp. 50-54, 1998).
Alternatively, a double-beam interference exposure method (holographic method) may be used to write a Bragg diffraction grating. A He-Cd laser (325 nm, 421 nm) and Ar laser (351 nm) may be used as the light source used for this exposure. Exposure by an Ar ion laser (514.5 nm) may be made possible by using a sol-gel liquid composition having photosensitivity at a visible range and adding a visible light sensitizer. In order to produce an embedded or ridge type optical waveguide by light irradiation, a double-bond portion sufficient for holographic exposure for the formation of a diffraction grating must be left in the film by controlling irradiation energy.
The cycle of the formed grating (diffraction grating) can be controlled by the interference angle of coherent laser light to be interfered in the case of interference exposure. Meanwhile, the above cycle can be controlled by the cycle of grooves in a phase mask in the case of a phase mask method. By forming a grating cyclic structure in the core portion of an optical waveguide, a reflective optical filter which selectively reflects light having a specific wavelength can be formed. It is known that there is the following relationship among the reflection light wavelength, refractive index and refractive index (grating) cycle of the optical filter.
xcex=2neffxc2x7xcex9
wherein neff is the effective refractive index in a mode for guiding an optical waveguide, and xcex9 is the change cycle of refractive index. When the photosensitive composition for forming a light transmitting material of the present invention is used to selectively reflect light having a wavelength of 1.55 xcexcm, xcex9 is 0.5 xcexcm because the refractive index is about 1.5. Thus, a narrow-band optical filter can be obtained. By changing the grating cycle A in an axial direction for guiding the light of the core portion of the optical waveguide, a wide-band optical filter having a chirped grating can be obtained.